1. Field of the Invention
The present invention relates to a viscous coupling for a vehicle and the like, which coupling utilises in action the viscous resistance of a viscous fluid employed therein.
As is well known, a conventional viscous coupling comprises first and second torque-transmitting members rotatably arranged relative to each other, the first and the second torque-transmitting members cooperating in defining an operating chamber at least partially filled with a viscous fluid, a plurality of first plates engaging the first torque-transmitting member to prevent relative rotation of the first plates and the first torque-transmitting member, and a plurality of second plates non-rotatably mounted on the second torque-transmitting member, the second plates being disposed alternately with the first plates in the operating chamber. The viscous resistance of the viscous fluid to rotation of one of the torque-transmitting members to the other of the torque-transmitting members provides a means for transmitting torque between the torque-transmitting members, which torque automatically varies so as to be substantially proportional to a difference in rotational speed between the torque-transmitting members
2. Description of Prior Art
The conventional viscous coupling of this type is disclosed in, for example such as Japanese Patent Laid-Open No. 186051/1988 (in Japanese, called Tokkaisho 63-186051), and employed as: a part of a power-transmitting unit interposed between a front-wheel axle and a rear-wheel axle of a four-wheel drive vehicle; or a limited slip differential interposed between opposite wheels of a front-wheel axle of a front-wheel drive vehicle.
When such a conventional viscous coupling is employed in a vehicle with an anti-skid brake system (ABS), the viscous coupling operates to transmit torque corresponding to a difference in rotational speed between a front and rear wheel axle of the vehicle or opposite wheels of each of the wheel axles of the vehicle even when the anti-skid brake system (ABS) of the vehicle is operated. As a result, the wheels of the vehicle interfere with each other in braking force to make it difficult precisely to control a brake fluid pressure of the vehicle, so that the anti-skid brake system (ABS) of the vehicle is prevented from operating normally. This is a problem inherent in the conventional viscous coupling of this type.
In order to resolve the above problem, another Japanese Patent Laid-Open No. 65918/1986 (called Tokkaisho 61-65918) discloses another conventional viscous coupling having a construction in which: an annular disc spring or Belleville spring is interposed between the first plates and/or the second plates so as to separate these plates from each other; and each of the plates is so constructed as to be movable against a resilient force exerted by the annular disc spring under the influence of an external fluid pressure applied to the operating chamber or an external mechanical force exerted by a mechanically controlled pushing element, so that each of the first and the second plates is moved to vary a distance between the adjacent ones of the plates, whereby a torque-transmitting characteristic of the viscous coupling is controlled. In the other conventional viscous coupling, the mechanically controlled pushing element is so constructed as to extend into the operating chamber through a housing of the viscous coupling so as to push the plates through a bearing in the operating chamber which is filled with the viscous fluid, usually a high viscosity silicone oil. However, the silicone oils substantially lack lubricating ability. In addition, the silicone oils are poor in cooling ability so that, in operation, temperatures of the silicone oils filled in the operating chamber of the viscous coupling considerably increase. Consequently, in such another conventional viscous coupling, the bearing mounted in the cooperating chamber filled with the silicone oils is subject to considerable wear in operation and, therefore, shortened service life, which lowers the reliability of the viscous coupling and increases the maintenance cost of the coupling. In addition, in the other conventional viscous coupling, a cylinder of the mechanically controlled pushing element of the viscous coupling is integrally formed with the housing which constitutes a rotary member of the viscous coupling. In case the mechanically controlled pushing element of the viscous coupling is operated by the fluid pressure, a swivel area must be provided in a fluid passage of the viscous coupling between the cylinder and a stationary portion of the coupling, which portion is mounted on the vehicle body. However, it is very difficult to realise a sealing engagement in such swivel area of the other viscous coupling. In other words, since the rotating parts of the viscous coupling vibrate in operation to have a tendency to cause a pressure leakage of the fluid, it is necessary for such swivel area of the fluid passage to employ a sealing means having a complex construction which increases the manufacturing cost of the viscous coupling. Furthermore, in the other conventional viscous coupling, since an actuator of the mechanically controlled pushing element is integrally formed with the housing, the housing which constitutes the rotary member of the viscous coupling gains in weight to increase its moment of interia. Therefore, such heavyweight housing is disadvantageous in controlling vibrations of the viscous coupling. In addition, in the other conventional viscous coupling, the actuator of the mechanically controlled pushing element is disposed in the housing side, and a pushing force exerted by the pushing element is received by a snap ring fixedly mounted on a hub side. Consequently, when the actuator is energised, contact surfaces, which are perpendicular to an axis of rotation of the viscous coupling and through which contact surfaces the housing and the hub are brought into contact with each other, are subjected to the pushing force exerted by the pushing element, and therefore, wear together so that the housing and the hub are poor in durability. In addition, since the operation conducted by the actuator is performed through the pushing element having been inserted into the operating chamber, a pressure of the viscous fluid filled in the operating chamber increases as the pushing element is moved against the resilient force exerted by the annular disc spring. Further, the pressure of the viscous fluid filled in the operating chamber also increases when a temperature of the operating chamber increases. When the pressure of the viscous fluid filled in the operating chamber increases, a force which is a product of a cross sectional area of the inserted portion of the pushing element and the pressure of the viscous fluid filled in the operating chamber acts on the pushing element so as to expel the pushing element from the operating chamber and, therefore, forms a resistance to the pushing force exerted by the actuator to make it difficult to precisely control the torque-transmitting characteristics of the other conventional viscous coupling. Consequently, in order to overcome such resistance to the pushing force exerted by the actuator, it is necessary for the other conventional viscous coupling to employ a large-sized actuator.
On the other hand, Japanese Patent Laid-Open No. 286836/1987 (called Tokkaisho 62-286836) discloses a conventional viscous coupling system having a construction in which: a viscous coupling is connected in series with a multiple disc clutch to permit or prevent torque to be or from being transmitted; a tightening or clamping force of the multiple disc clutch is so controlled as to control a torque-transmitting characteristic of the whole system; and the multiple disc clutch also serves as a torque limiter. In the above system, since the multiple disc clutch forms a separate unit which is connected in series with the viscous coupling, the whole system has a large dimension in a direction of an axis of rotation of the system, and is very heavy in weight and complex in construction.
Another example of a viscous coupling is disclosed in DE 3834555C. In this previously proposed coupling the plates of one of the sets of plates are not themselves rotationally fast with the respective torque-transmitting member, but have interposed between them rings which are rotationally fast with the member. An actuator is operable on a pressure member to press the plates and rings into frictional engagement with one another, so that the coupling can transmit torque. The arrangement has a disadvantage in that the pressure member causes a change in the volume of the operating chamber of the coupling, and the internal pressure in the chamber influences the force exerted by the pressure member on the plates and intervening rings, changing the torque-transmitting characteristics of the coupling.